This invention relates to a method and composition for preventing the fouling of submerged objects or marine structures while also minimizing pollution, and more particularly to a method and composition for preventing fouling of marine structures for an extended period of time by using organotin compounds which are chemically bonded to synthetic polymers.
From the beginning of man's attempt to use water to travel, he has been plagued with the problem resulting from the fouling of ships, buoys, pilings, and other types of marine structures, by organisms present in the water. It has been found that microorganisms, their viscous bio-organic products and absorbed organic matter, constitute a tenacious, opaque slime which forms on these submerged surfaces. The initial organisms in this fouling sequence are bacteria followed by a biotic progression of diatoms, hydrids, algae, bryozoans, and finally macrofoulants. Macrofoulants tend to be rugophilic, settling on roughened surfaces in preference to smooth surfaces. It is thought that primary marine slimes precondition the submerged surface in some manner stimulating the settling of macrofaulants. This theory is supported by the fact that barnacle settlement is less frequent on clean glass surfaces compared to those covered with emollient films high in particular matter. This film may provide a physical substrate and/or a nutritive source which encourages the attachment of macroscopic plants and animals.
The resultant effect of the concentration of these plants and animals settling and attaching themselves to ships is that they contribute significantly to speed reduction, they increase fuel consumption, and in the area of concern over water craft detection, they strengthen the noise signature of vessels under way thereby rendering covert activity more difficult.
The problem of fouling applies not only to vessels but also to other marine structures. For example, fouling of sonar domes has been found to seriously limit the active and passive modes of operation of ships' acoustical systems. Fouling of moored data systems and ship-and-shore facilities by marine organisms impedes operations and necessitates a large maintenance allocation. Buoys can shift due to the excessive weight of fouling organisms. Wood pilings in berthing facilities undergo structural weakening and ultimate destruction due to marine borer and fungal attack. The fouling of piping including steel piping and bronze couplings and fittings in the sea-water intake piping systems of ship-and-shore facilities leads to reduced flow rates, valve seat damage, and accelerated metal corrosion. Concrete of ferro-cement or other similar structures are also adversely affected by fouling organisms.
It is only since the beginning of this century that improvements have been made in the early Phoenician methods of using copper cladding and poisonous paints to prevent fouling. One such improvement involved the use of asphalt as an antifouling coating. Another improvement involved the use of coating containing copper salts or oxides. In addition, organometallic salts, e.g., tri-n-butyltin oxide (TBTO), tri-n-butyltin fluoride (TBTF), tri-n-butyltin sulfide (TBTS), being extremely powerful biocides and toxic to a wide range of marine organisms, have been used as the active ingredients in a variety of antifouling coatings. Investigations into the use of organotin compounds for use in antifouling paints have received much attention because coatings containing these compounds exhibit excellent pigment retention, but do not accelerate the corrosion of metal substrates.
However, these and other state-of-the-art compositions possess several drawbacks which limit their use as effective antifoulants. Asphalt lacks the desired durability to make it an effective answer to the problems posed. Other existing antifouling coating systems involve the use of paints which typically contain sufficient water soluble pigments, metal salts and inert fillers for direct contact to occur between the particles within the paint film; as one particle dissolves, another in contact with it is exposed to solvolysis. This process, called leaching, is uncontrolled and varies with such factors as coating age, water velocity, temperature and salinity, and the primary slime layer. Quantitative information indicates that in most cases the leaching rate of antifouling paints is excessive and poses a potential environmental hazard. As a result, the best available antifouling coatings are inefficient and short lived because of the above-mentioned leaching process. This inefficiency leads to the concentration of the antifouling agent in the water in quantities well above normal oceanic background.
Although concern over avoiding a potential polution hazard was not a motivating factor, attempts have been made in the past to incorporate the toxic substance in a polymeric antifouling coating composition by chemically bonding the toxic ingredient to the polymer. For example, see Leebrick, U.S. Pat. No. 3,167,473, or Goto et al., U.S. Pat. No. 3,684,752. However, this type of antifouling compositions has not proved to be commercially successful, apparently because of the inability of the resulting coatings to maintain their integrity over an extended period of time.
Thus, after approximately 12 to 20 months, or 50,000 miles transit the presently available antifouling paint systems begin to foul, which is indicative of the depletion of most of the antifouling agent from the coating into the marine environment, or of a complete breakdown of the coating itself. This short performance time is far less than the life time of 5 years or more desired of an antifouling coating.
The leaching rate of metallic salts and organometallic salts from presently used antifouling coating systems is governed by the relative proportions and solubilities of three components: rosin, antifouling agent and pigment. Rosins are resinous organic acids which have a water solubility of 100 mg/cm.sup.2 /day. In addition to a relatively high solubility, rosins are consumed by sliming marine bacteria. This results in an accelerated biodegradable action, thus adding to the breakdown of the coating and subsequent accelerated release of metallic and organometallic salts. At present the primary antifouling agents used by the U.S. Navy are cuprous oxide which has a water solubility of 0.5 mg/l and tributyltinfluoride which has a water solubility of 2.9 mg/l. Leaching of inorganic and organometallic antifouling salts from coating formulations could possibly be reduced by using their less water-soluble homologs in conjunction with insoluble pigments and as little rosin as possible. However, state-of-the-art antifouling technology has not provided an effective antifouling composition having a controlled leaching rate which minimizes the presence of toxic antifouling agents in the marine environment. Hence, it would be desirable to provide a new class of effective antifouling compositions having low leaching rates as compared to hitherto available compositions.